Problem repairing manual transmission

Hi everyone, I'm new here. I'm from Argentina.. so I will try my best writing and explaining my problems in english 🙂 But first of all.. just thank u.

Well, I have a Audi a3 1.4t 2012 with manual transmission. Aprox 100.000 kms.
The point is that a few months ago, the transmission started to droning. Everytime. And it get louder and louder so I took it to my mechanic to fix it.
Apparently the problem was that the transmission oil wasnt correctly changed and it caused that problem.
This mechanic changed some "platalet" and a bearing on the 5th shift.

The problem comes here.. when he gave the car back to me the transmission stopped droning and works good, BUT it have a problem.
When i'm using the car in 5th shift.. the on board computer doesnt get the signal so it doesnt show that i'm runing on that shift.
It causes some problems.. for example: cruise control doesnt works on 5th shift because the computer loses the signal when it try to accelerate. But it works good on all the other shifts.

Does anyone have an idea of what is causing this problem?

Hope u can help me guys. and thank u for reading this.
Octavio